Hello!
Condo, small room. All walls and ceiling are adjacent to my own rooms.
Floor is above a quiet neighbor and I am trying to protect her from my new piano.
It was a big expensive undertaking and now we discover that it has failed.

Steps already taken:
1. Hardwood floor ripped out. 12mm Ecore underlayment installed, with Bostik sound-deadening glue above and below it. New hardwood floor on top.
2. 8x10 rug covering majority of room, with thick memory-foam rug pad.
3. Piano placed on caster cups designed to isolate vibrations from floor.
After all this expensive work was done, We tested it again. The sound of the piano is still very present in the downstair's neighbor's room. I am absolutely dejected.
Voices did not transmit, and or course the lower tones transmitted most of all.
So this brings me to my present question:
There is a large window seat right next to the low end of the piano, which was not addressed. Soundproofing expert returned and said that the window seat cavity may be hollow. He recommended that I open it up, fill it with a couple layers of mineral wool, and re-install that flooring.
Will this work?
It's so discouraging to have all this expensive work done, and to wait the many months for it to get done, and then to have it seem to make almost no difference.
Attached is a photograph from the side of the window seat, whose top is about 18"x74".
I earnestly seek your advice, and I thank you in advance for your consideration.